The present invention relates generally to multiband antenna systems for use in motor vehicles, and more particularly to a multiband antenna system suitable for reception of AM/FM radio signals and further for quasi-microwave communication, for example.
In order to meet the recent requirement for using motor vehicles as mobile stations, various types of multiband antenna systems suitable for quasi-microwave communication (radiotelephony) concurrent with reception of AM/FM radio signals have been proposed as exemplified by Japanese Patent provisional Publication Nos. 62-179202 and 61-502579. FIGS. 1A and 1B show a basic arrangement of the multiband antenna system disclosed in the aforementioned Japanese Patent Provisional Publication No. 62-179202. As illustrated in FIGS. 1A and 1B, the prior art multiband antenna system is of the cellular telephone type for use in motor vehicles and comprises a first antenna element 1 having a length of one-quarter wavelength at the telephone band center frequency and acting as a monopole antenna unit in association with vehicle ground plane 11, a phasing coil 2 having an electrical length of three-half wavelength or one-half wavelength at the telephone band center frequency, a second antenna element 3 having a length corresponding to one-half wavelength at the telephone band center frequency and functioning as a dipole antenna unit, a third antenna element 5 and another coil 4 adapted to resonate at the telephone band center frequency so as to prevent currents inducted in the aforementioned first antenna element 1, phasing coil 2 and second antenna element 3 from flowing into the third antenna element 5, whereby the entire gain is increased up to the standard dipole antenna ratio 3dB. In addition, for reception of the radio signals, the conventional antenna system resonates with respect to the FM band center frequency with the entire physical length of the respective antenna elements and the respective coils and further acts as a capacitor antenna for the AM band with the apparent length of the respective antenna elements and the respective coils. The respective currents caused thereby are mixed and introduced through a coaxial tube 6, encased in an antenna encasing tube 12 and a feeder distribution center 7 into a branching filter 8 having a telephone terminal 9 and an AM/FM terminal 10. Illustrated at numeral 14 is a motor system which allows extension and retraction of the antenna unit from and into the antenna encasing tube 12.
FIG. 2 illustrates a basic arrangement of the prior art multiband antenna system disclosed in the above-mentioned Japanese Patent Provisional Publication No. 61-502579. The multiband antenna system comprises a cellular telephone antenna element 20 including a sleeve antenna section having a length equal to one-half the wavelength at the telephone band center frequency and a radio wave reception antenna element 21 having a length equal to one-quarter of the wavelength at the FM band center frequency and acting as a monopole antenna by protruding from a vehicle body ground plane 22. The telephone antenna element 20 and the radio wave reception antenna element 21 are telescopically arranged, and in the extension state, the telephone antenna element 20 is provided at the upper end of the radio wave reception antenna element 21 in order to eliminate electrical interference from the vehicle body 22. A feed cable 23, together with an extention and retraction cable 25, extends through the inside of the radio wave reception antenna 21 and an antenna encasing tube 24 and is taken up by a take-up section 27 of a motor driving mechanism 26 and coupled through a telephone output terminal 28 to a telephone terminal 29. Further, the radio wave reception antenna element 21 is electrically connected to an inner conductive tube 30 located in the antenna encasing tube 24 and coupled to a radio receiver 32 through a radio output terminal 31 provided at a portion of the antenna encasing tube 24.
An important problem in such multiband antenna systems constructed by a combination of independent antenna elements relates to deterioration of the antenna performance due to the mutual synergistic interference. One possible solution is that the independent antenna elements are arranged up and down in insulating relation to each other so as to eliminate the interference to each other or that a concentrated constant circuit is provided therebetween to prevent unnecessary leakage currents. However, this arrangement causes a complex structure and difficulty of the characteristic adjustment so as to lower the productivity.